New paper: effective selection in ancient asexuals

Biologists from Goettingen and Lausanne analyse how loss of sex affects the genetic makeup of oribatid mites

Sexual reproduction is linked to various benefits for the survival of a species. Amongst others, sex counters the accumulation of accidentally occurring deleterious mutations by allowing for exchange of genetic material. According to the scientific consensus, loss of sex in favour of clonal reproduction (there are only asexual mothers that give birth to asexual daughters) leads to a reduced effectiveness of selection and a stepwise accumulation of deleterious mutations, which eventually causes extinction of a species. However, experimental evidence is scarce and restricted to species that lost sex in recent times.

To unravel the consequences of loss of sex in the long term, scientists of the Universities of Göttingen and Lausanne have sequenced and bioinformatically analysed the genetic makeup of various sexual and ancient asexual oribatid mites. The findings are surprising: Ancient asexual oribatid mites are more effective in purging deleterious mutations as compared to their sexual relatives. The results were reported in Nature Communications (DOI 10.1038/s41467-017-01002-8).

“Oribatid mites are well suited for analysing the long-term consequences of asexual reproduction, as within this animal group, sex was lost millions of years ago several times independently.” explains Dr. Jens Bast, research fellow at the University of Lausanne and senior author of the study. “The discovery of asexual oribatid mites purging deleterious mutations more effectively as compared to their sexual relatives indicates the existence of a certain peculiarity in oribatid mites which enables them to survive without sex throughout evolutionary time scales.” adds Alexander Brandt, first author of the study and doctoral student in the animal ecology group of Prof. Dr. Stefan Scheu at the University of Göttingen.

So far, scientists can only speculate about the nature of the peculiarity: the population sizes of oribatid mites may play a role. The number of animals that make a population has, just like mixing up the genomic makeup, a strong influence on how effective natural selection counters the accumulation of deleterious mutations. The fact that populations of asexual oribatid mites are on average substantially larger as compared to sexual species indicates that the observed pattern might indeed be due to their very large populations. This allowed at least some species of oribatid mites to survive without sex for millions of years.

The asexual oribatid mite species Nothrus palustris on litter particles, which measures about 0.9 mm in length.